Effects of stimulus waveform on transcranial magnetic stimulation metrics in proximal and distal arm muscles

The purpose of this study was to determine the effect of common transcranial magnetic stimulation (TMS) waveforms (monophasic and biphasic) on resting motor threshold (RMT), active motor threshold (AMT), and motor evoked potential (MEP) amplitudes in the biceps and first dorsal interosseous (FDI) because waveforms may affect motor targets differently. We also determined test-retest reliability.

Ten individuals participated in two sessions of TMS delivered to the motor cortex. Monophasic stimulation to induce a posterior-anterior current in the brain (mono_PA) and biphasic posterior-anterior then anterior-posterior (bi_PA-AP) were applied in each session in random order. In each session, there were four blocks of measurements (2 muscles × 2 waveforms) of RMT, AMT and MEPs at the hotspot location. MEPs were normalized to the maximum EMG signal.

RMTs and AMTs were lower for mono_PA compared to bi_PA-AP stimulation for the biceps (p<0.01) and FDI (p<0.01). Normalized MEPs were greater for mono_PA compared to bi_PA-AP stimulation in the FDI (p=0.01) and not different in the biceps (p=0.86). Motor thresholds were not different between sessions suggesting high reliability (p<0.01). Normalized MEPs had very low reliability across sessions in the FDI, and moderate reliability in the biceps.

Preliminary investigation suggests the effect of TMS waveform on motor thresholds is similar in upper limb proximal and distal muscles, but the effect differs per motor target for MEPs. Further, test-retest reliability of waveform effects was sensitive to target muscle. These findings may contribute to improve the efficacy and reliability of TMS for clinical use.